Diffusion mr imaging with fat suppression

ABSTRACT

The invention relates to a fat suppressed diffusion image determination apparatus, a corresponding method and a corresponding computer program, for determining a diffusion weighted magnetic resonance image (DWI) of an object (10), the fat suppressed diffusion image determination apparatus (100) comprising: a diffusion reference image providing unit (110) for providing a diffusion reference MR image of the object (10), a fat image determination unit (120) for determining a fat image from the diffusion reference MR image, a diffusion weighted image providing unit (130) for providing a diffusion weighted MR image of the object, a fat suppressed image determination unit (140) for determining a fat suppressed diffusion weighted MR image using a combination of the diffusion weighted MR image and the fat image. The invention allows for a robust fat suppression in diffusion MRI with improved SNR and scan time trade-off.

FIELD OF THE INVENTION

The invention relates to the field of magnetic resonance (MR) imaging.It concerns a fat suppressed diffusion image determination apparatus anda corresponding method for determining a diffusion weighted magneticresonance image (DWI) of an object. The invention also relates to a MRIsystem and to a computer program to be run on the fat suppresseddiffusion image determination apparatus.

BACKGROUND OF THE INVENTION

Image-forming MR methods which utilize the interaction between magneticfields and nuclear spins in order to form two-dimensional orthree-dimensional images are widely used nowadays, notably in the fieldof medical diagnostics, because for the imaging of soft tissue they aresuperior to other imaging methods in many respects, do not requireionizing radiation and are usually not invasive.

Diffusion-weighted MRI (DWI) has been adopted widely in clinicalpractice, for instance to improve the detection of malignancy for a widevariety of cancers. Fat suppression is an essential ingredient in DWI asthe pixel-wise apparent diffusion coefficient (ADC) is a correlate forpathology, in particular the grading of cancer. Relative to water, thediffusivity of fat is extremely low and any residual signal willconfound the assessment of the ADC of the calculation or computed DWI(cDWI) images.

One approach to fat suppression in diffusion MRI is to exploit thechemical shift between fat and water using spectrally-selective pulsessuch as used in SPectral Attenuated Inversion Recovery (SPAIR) or byapplication of a selective inversion-recovery pulse scheme such asShort-TI Inversion Recovery (STIR) as known in the art. These schemesare possibly augmented by inverted slice-select gradients for the90°-180° pulse pair to shift residual fat out of the spectral refocusingband.

Each of these methods come with a loss of signal to noise ratio (SNR),which is inherently low in DWI. Also, the fat signal is composed ofmultiple spectral peaks, primarily at 3.5 ppm and at 1 ppm, wherein 1ppm is too close to the water line to be suppressed byspectrally-selective excitation.

In other imaging sequences apart from diffusion imaging, algorithmicseparation of water and fat signals such as the DIXON algorithm hasdeveloped as an alternative to spectral fat suppression. However, it isdifficult to apply the multi-echo time DIXON algorithm to diffusionimaging, why alternative methods are needed.

Burakiewicz et al. (DOI: 10.1002/mrm.25191, 2014) disclose the potentialof DIXON DWI with multiple echo-time shifted single shot echo planarimaging (EPI) acquired images. This method comes at the expense of alonger echo time which thereby leads to a further reduced SNR. Further,the publication showed to require a phase navigator to ensure that thetwo or three acquisitions at different echo times can be related to thesame phase since the phase in DWI is extremely sensitive to deviationsfrom the motion encoding during the diffusion gradients. Further, thesingle-shot acquisitions are replaced by multiple acquisitions at eachvalue of the diffusion parameter (b-value) which increases the scan timeby a factor of up to 3.

Larkman et al. (ISMRM 2005, 505) disclose the application of SENSitivityEncoding (SENSE) to generate water and fat resolved images based on asingle shot EPI diffusion weighted image. However, the scheme isdescribed as, while being effective, not being optimal as all pixels areconsidered to be two fold degenerate independent of the presence of fator not. Thus, a degeneration of the SNR across the whole image due tog-factor effects is experienced even at low SENSE reduction factors.Thus, the method is not compatible with high SENSE factors which arenormally required for geometrically-accurate EPI.

Magnetic resonance in medicine, vol 79, no. 1, 5 Mar. 2017, pages152-159 discloses an improved olefinic fat suppression in skeletalmuscle DTI using a magnitude-based dixon method.

SUMMARY OF THE INVENTION

It has thus been an object of the present invention to provide a fatsuppressed diffusion image determination apparatus, a corresponding fatsuppressed diffusion image determination method, a MR imaging system andcorresponding computer program that allow for a robust fat suppressionin diffusion MRI with improved SNR and scan time trade-off.

In a first aspect of the invention, a fat suppressed diffusion imagedetermination apparatus for determining a diffusion weighted magneticresonance image (DWI) of an object is provided. The fat suppresseddiffusion image determination apparatus comprises:

-   -   a diffusion reference image providing unit for providing a        diffusion reference MR image of the object,    -   a fat image determination unit for determining a fat image from        the diffusion reference MR image,    -   a diffusion weighted image providing unit for providing a        diffusion weighted MR image of the object,    -   a fat suppressed image determination unit for determining a fat        suppressed diffusion weighted MR image using a combination of        the diffusion weighted MR image and the fat image.

Since the fat suppressed diffusion weighted MR image is determined basedon the fat image, which itself is determined based on a different image,namely the diffusion reference MR image, no additional fat imageacquisitions for the diffusion weighted MR image are necessary. Thus,the fat suppressed diffusion image determination apparatus according tothe invention allows to determine the fat suppressed diffusion weightedMR image without deteriorated SNR, even if, for instance, high SENSEreduction factors required for geometrically accurate EPI are employed.

The fat image can be data provided in image space. However, the fatimage is not limited to values or data in image space and can also berepresented in, for instance, k-space.

The invention is based on the finding that the separation of water andfat is independent of the diffusion encoding and that further, sincemobility of fat may generally be neglected, the fat signals, i.e. thefat image, is independent from the diffusion encoding. Thus, the fatcontributions can be eliminated from the diffusion weighted MR image byconsidering the fat image determined based on the diffusion reference MRimage.

Further, since no additional fat acquisitions are required fordetermining the diffusion weighted MR image, scan time is not increasedfor obtaining the diffusion weighted MR image.

The object is preferentially a living being, i.e. a person or an animal,or a part of the living being such as an organ like the liver, thebrain, the heart, the lung, the pancreas, the kidney, et cetera. Theobject can also be a technical object in which diffusion occurs.

The diffusion reference image providing unit can be a storage in whichthe diffusion reference image data are stored and from which thediffusion reference MR image data can be retrieved for providing thesame. The diffusion reference image providing unit can also be areceiving unit for receiving the diffusion reference image data and toprovide the received diffusion reference image data. For instance, thediffusion reference image providing unit can be adapted to receive thediffusion reference image data from an MR data acquisition device in araw or processed form, i.e. in the form of a reconstructed MR image. Thediffusion reference image providing unit can also be a MR dataacquisition device of a MR imaging system.

Likewise, the diffusion weighted image providing unit can be a storagein which the diffusion weighted image data are stored and from which thediffusion weighted image data can be retrieved for providing the same.The diffusion weighted image providing unit can also be a receiving unitfor receiving the diffusion weighted image data and to provide thereceived diffusion weighted image data. For instance, the diffusionweighted image providing unit can be adapted to receive the diffusionweighted image data from an MR data acquisition device in a raw orprocessed form, i.e. in the form of a reconstructed MR image. Thediffusion weighted image providing unit can also be a MR dataacquisition device of a MR imaging system.

Preferentially, the diffusion reference MR image originates from a MRimage acquisition with no or insignificant diffusion weighting. Morepreferably, a diffusion parameter which is indicative of strength andduration of the diffusion gradients is significantly lower for thediffusion reference MR image than for the diffusion weighted MR image.The diffusion parameter is preferably the b-value described in, forinstance, Le Bihan et al. “MR imaging of intravoxel incoherent motions;application to diffusion and Perfusion in neurologic disorders.”,Radiology. 161: 401-407 (1986).

The diffusion reference MR image preferentially comprises raw acquireddata, for instance in k-space, partly reconstructed image data, e.g. analiased or folded image in case of parallel MRI, and/or the fullyreconstructed image data. Preferably, the diffusion reference MR imageacquisition timing is identical with the timing for the diffusionweighted MR image acquisition, with the diffusion-encoding parameter bset to zero by applying zero gradient area for the diffusion-encodinggradients, or a b-value near zero, i.e. a relatively low b-value. In thefollowing, the terms diffusion reference MR image and non-diffusionweighted MR image will be used synonymously, wherein a non-diffusionweighted MR image includes MR images with low or insignificant diffusionweighting, as exemplified above.

Contrary to the diffusion reference MR image, the diffusion weighted MRimage originates from a MR image acquisition having the diffusionparameter b significantly larger than 0, for instance 100 or 1000 s/mm²,wherein this b-values are of course exemplary and not limiting. Also thediffusion weighted MR image can comprise raw acquired data, for instancein k-space, partly reconstructed image data, e.g. an aliased or foldedimage in case of parallel MRI, and/or the fully reconstructed imagedata.

In a preferred embodiment of the fat suppressed diffusion imagedetermination apparatus, the diffusion reference image providing unit isconfigured to provide the diffusion reference MR image acquired with adiffusion parameter of at most 200 s/mm², preferably at most 100 s/mm²,and most preferably at most 50 s/mm². The diffusion parameter notexceeding these limits has the advantageous effect to efficientlysuppress flow effects.

In a further preferred embodiment of the fat suppressed diffusion imagedetermination apparatus, a difference between the b-values of thediffusion reference MR image and the diffusion weighted MR imageacquired is at least 100 s/mm². However, the inventive concept can beadvantageously applied to significantly larger differences in b-values,such as in the range of 500 s/mm², 1000 s/mm², 2000 s/mm² or more.

In a preferred embodiment of the fat suppressed diffusion imagedetermination apparatus, the diffusion weighted image providing unit isconfigured to provide a plurality of diffusion weighted MR images of theobject with respective different diffusion parameters, wherein the fatsuppressed image determination unit is configured to provide a pluralityof fat suppressed diffusion weighted MR images for each of the pluralityof diffusion weighted MR images using the fat image.

Advantageously, only one fat image is required, wherein this fat imageis used for determining multiple fat suppressed diffusion weighted MRimages having different diffusion encoding factors (b values).Accordingly, the acquisition time can advantageously be reduced, leadingto a shorter scan time. Even if the diffusion reference image, which isused for determining the fat image, is acquired having a longer scantime, the overall scan time will be reduced since the increase of scantime only applies to the diffusion reference MR image, while a pluralityof the images, i.e. the diffusion weighted MR images, are acquiredwithout the increased scan time.

In a preferred embodiment of the fat suppressed diffusion imagedetermination apparatus, the diffusion reference image providing unitand the diffusion weighted image providing unit are configured toprovide the MR images using a parallel imaging method, respectively.

Parallel imaging methods are known in the art and include the conceptsof having multiple coils, which each have a different sensitivity todifferent regions of the image space. The use of multiple receiver coilshas reduced acquisition times significantly. An overview over parallelmagnetic resonance imaging techniques can be found in, for instance,Larkman et al. “Parallel magnetic resonance imaging” Phys. Med. Biol. 52(2007) R15-R55. In particular in diffusion MR imaging, parallel imagingensures the echo time to be short enough to yield valuable SNR signals.

In a preferred embodiment of the fat suppressed diffusion imagedetermination apparatus, the parallel imaging method comprises at leastSENSE.

SENSE is known, for instance, from Pruessmann et al. “SENSE: Sensitivityencoding for fast MRI.” Magn. Reson. Med. 1999; 42:952-962, in additionto the review on parallel MR imaging cited above. The exemplary parallelimaging techniques of this embodiment are primarily performed in imagespace after reconstruction of data from the individual coils. Otherapproaches including GRAPPA/ARC methods operate primarily on k-spacedata before image reconstruction and are also contemplated to beemployed in the alternative. Further, it is clear that every known andfeasible modification to the parallel imaging methods described aboveare likewise contemplated.

In the particularly preferred embodiment of SENSE, for instance, thereconstructed image data of the different coils shows aliasing, i.e. isfolded, and is to be unfolded before the final image is obtained, whichis precisely performed by the SENSE algorithm.

In a preferred embodiment of the fat suppressed diffusion imagedetermination apparatus, the diffusion reference image providing unit isconfigured to provide a folded representation of the diffusion referenceMR image of the object, wherein the fat image determination unit isconfigured to determine an unfolded fat image by decomposing fat andwater from the representation of the diffusion reference MR image, andto determine a folded representation of the fat signals as the fatimage.

Preferentially, the fat image can then be subtracted from the foldedrepresentation of the diffusion weighted MR image, i.e. the image aftertransformation from k-space but before reconstruction. The decompositioncan be done in a known way based on, for instance, an a priori knownchemical shift induced displacement between fat and water. Water and fatseparation in EPI is known from, for instance, Larkman et al. Proc.Intl. Soc. Mag. Reson. Med. 13 (2005) 505.

In a preferred embodiment of the fat suppressed diffusion imagedetermination apparatus, the fat image determination unit is configuredto determine a complex valued fat image and the diffusion referenceimage providing unit is configured to provide a complex valued diffusionreference MR image. Additionally or alternatively, also the diffusionweighted image providing unit can provide the fat suppressed diffusionweighted MR image complex valued. In particular, the provision of the MRimages preferentially comprises a complex value EPI reconstruction.

Further preferentially, both the folded or SENSE reduced representationof the diffusion reference MR image and the unfolded fat image areprovided as complex valued image data, which provide phase information.

In a preferred embodiment of the fat suppressed diffusion imagedetermination apparatus, the diffusion reference image providing unit isconfigured to provide the diffusion reference MR image using multipleshots for covering the entire k-space, wherein the multiple shots havesimilar k-space trajectories, respectively, wherein the k-spacetrajectories of the multiple shots have a respectively different shiftin phase encoding direction.

In contrast to single shot acquisition, k-space is traversed in thisembodiment using a plurality of radio frequency (RF) excitation pulses.The k-space trajectories of the multiple shots of the diffusionreference image correspond preferentially to the k-space trajectoriescovered by the diffusion weighted MR image in order to assure that thegeometric deformation in each shot is the same.

In a preferred embodiment of the fat suppressed diffusion imagedetermination apparatus, the diffusion reference image providing unit isconfigured to perform a multi-shot acquisition for the diffusionreference MR image. The multi-shot acquisition provides a betterconditioning of the reconstruction problem and thus at least partiallyalleviates the need to resolve the aliasing due to undersampling for thereconstruction of the diffusion reference MR image, with its real andimaginary signal components.

In a preferred embodiment of the fat suppressed diffusion imagedetermination apparatus, the diffusion reference image providing unit isthus configured to perform an echo planar imaging (EPI) reconstructionto complex valued image data.

This complex valued diffusion reference MR image data is the input for aSENSE reconstruction to derive the diffusion suppressed water image andthe fat image. This alleviates the need for multi echo time (TE)acquisitions.

In a preferred embodiment of the fat suppressed diffusion imagedetermination apparatus, the fat image determination unit is configuredto determine the fat image using a SENSE separation of water and fatusing the EPI reconstructed complex valued image data.

The SENSE separation of water and fat is known from Larkman et al. citedabove. However, the SENSE separation is of course only one suitablemethod for water and fat separation while other separation algorithmsknown in the art can be employed instead.

In a preferred embodiment of the fat suppressed diffusion imagedetermination apparatus, the diffusion reference image providing unit isconfigured to provide the diffusion reference image with a particularSENSE reduction factor.

In this respect, it should be ensured that geometric distortion be thesame for each shot of a multi-shot acquisition. In essence, thiscondition can be met by ensuring a constant ratio of a differencebetween two adjacent lines in k-space to the echo spacing, i.e. thedistance in time between subsequent echoes.

In a preferred example, the sampled field of view (FOV) and the SENSEreduction factor can be increased through multiplying with factors,until the resulting SENSE factor is an integer. However, also otheroptions to meeting the constant geometric distortion requirement are ofcourse feasible.

To provide a further example, the diffusion reference MR imageacquisition can be configured to have a number of shots Ns equal to aSENSE reduction factor R of the diffusion weighted MR image acquisitionor corresponding to the nearest integer larger than the reduction factorR, which can be expressed as Ns=ceil(R).

Since the difference or delta between lines in k space must be identicalfor a SENSE reduced shot and for the multi-shot acquisition in order tokeep the distortions identical, less k-space lines per shot are neededto cover the same field of view in case the number of shots has beenrounded up, as will be described in the following. The number of phaseencoded k-space lines for a SENSE encoded shot or acquisition can bedefined as Nk(SENSE)=N/R, i.e. a ratio of an image resolution or numberof non-reduced phase encoded k-space lines N to the reduction factor R.

The image resolution N is the same for the multi-shot acquisition withNs shots, which leads to a number of k-spaces lines per shot Nk(pershot)=N/Ns, which is smaller or equal to the number of k-spaces linesfor the SENSE reduced shot Nk(SENSE). Thus, keeping the EPI train timingidentical, also non-integer reduction factors R for the MR imageacquisitions can be implemented by increasing the number of shots in thediffusion reference MR image acquisition.

It will be appreciated that applying water/fat separation inreconstruction, generally doubles the number of unknowns which, inparticular in a SENSE-undersampled image, e.g. the diffusion referenceMR image, results in a more prominent g-factor penalty. To alleviatethis penalty, a better determined reconstruction is preferentiallyensured. In an embodiment, the reconstruction can be additionallyconditioned by integrating a regularization parameter, which is capableof indicating where the fat and/or water is.

In a preferred embodiment of the fat suppressed diffusion imagedetermination apparatus, the fat image determination unit is configuredto add multiple averages of the diffusion reference image to the fatimage.

In this embodiment, it is assumed that the SENSE reconstruction problemis sufficiently well-conditioned, i.e. that the g-factor is sufficientlylow. The multiple averages, which can be in the range of 1 to 5 or more,will then be sufficient to address about a 10-20% signal to noise ratio(SNR) loss due to an increased g-factor penalty. Advantageously, thisapproach of adding a few averages can provide a reasonable estimate ofthe fat signal, i.e. the fat image, while being simple to implement.

In a preferred embodiment of the fat suppressed diffusion imagedetermination apparatus, the diffusion weighted image providing unit isconfigured to provide the diffusion weighted MR image before unfolding,wherein the fat suppressed image determination unit is configured tosubtract the folded fat image from the folded diffusion weighted MRimage and to determine the fat suppressed diffusion weighted MR image byunfolding the fat image subtracted folded diffusion weighted MR image.

Since the fat suppressed diffusion weighted MR image is unfolded in theform of a water only processing, i.e. after the fat image has beensubtracted from the folded diffusion weighted MR image, ill-conditioningof the reconstruction problem is prevented. In addition, no need for aphase navigator or the like arises since a single acquisition at eachvalue of the diffusion parameter b is sufficient and no need exists toparticularly consider the phase in diffusion-weighted imaging, which isextremely sensitive to deviations from the motion encoding during thediffusion gradients.

In a further aspect of the invention, a magnetic resonance imaging (MRI)system for imaging an object is provided. The MRI system comprises a MRimaging apparatus configured to acquire a diffusion reference MR imageand a diffusion weighted MR image of the object, and a fat suppresseddiffusion image determination apparatus according to claim 1.

Advantageously, the MRI system according to this aspect allows thedetermination of a diffusion weighted image (DWI) with the sameadvantages as described with reference to the fat suppressed diffusionimage determination apparatus above. Likewise, all preferred embodimentsof the fat suppressed diffusion image determination apparatus can becombined with the MRI system according to this aspect.

Preferentially, the diffusion reference image providing unit and thediffusion weighted image providing unit of the fat suppressed diffusionimage determination apparatus are configured to provide the diffusionreference MR image and the diffusion weighted MR image, respectively,acquired by the MR imaging apparatus.

Preferentially, the MRI system comprises a processing unit such as ahost computer wherein one, more or all of the units of the fatsuppressed diffusion image determination apparatus are realized as unitsof hard- and/or software included in the host computer. Additionally oralternatively, one, more or all of the units of the fat suppresseddiffusion image determination apparatus can be implemented remote fromthe MR imaging apparatus, such as at a remote server.

In a further aspect of the invention, a fat suppressed diffusion imagedetermination method for determining a diffusion weighted magneticresonance image (DWI) of an object is provided. The fat suppresseddiffusion image determination method comprises:

-   -   providing a diffusion reference MR image of the object,    -   determining a fat image from the diffusion reference MR image,    -   providing a diffusion weighted MR image of the object,    -   determining a fat suppressed diffusion weighted MR image using a        combination of the diffusion weighted MR image and the fat image

In a further aspect of the invention, a computer program for controllinga fat suppressed diffusion image determination apparatus as defined inclaim 1 is provided. The computer program comprises program code meansfor causing the fat suppressed diffusion image determination apparatusto carry out the fat suppressed diffusion image determination method asdefined in claim 14, when the computer program is run on the magneticresonance imaging apparatus.

It shall be understood that the fat suppressed diffusion imagedetermination apparatus of claim 1, the magnetic resonance imagingsystem of claim 13, the fat suppressed diffusion image determinationmethod of claim 14, and the computer program of claim 15, have similarand/or identical preferred embodiments, in particular, as defined in thedependent claims.

It shall be understood that a preferred embodiment of the presentinvention can also be any combination of the dependent claims or aboveembodiments with the respective independent claim.

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following drawings:

FIG. 1 shows schematically and exemplarily a MR imaging systemcomprising the fat suppressed diffusion image determination apparatusaccording to the invention; and

FIG. 2 shows schematically and exemplarily a flowchart of a fatsuppressed diffusion image determination method according to theinvention.

DETAILED DESCRIPTION OF EMBODIMENTS

According to the MR method in general, the body of the patient to beexamined is arranged in a strong, uniform magnetic field B₀ whosedirection at the same time defines an axis (normally the z-axis) of thecoordinate system to which the measurement is related. The magneticfield B₀ produces different energy levels for the individual nuclearspins in dependence on the magnetic field strength which can be excited(spin resonance) by application of an electromagnetic alternating field(RF field) of defined frequency (so-called Larmor frequency, or MRfrequency).

From a macroscopic point of view, the distribution of the individualnuclear spins produces an overall magnetization which can be deflectedout of the state of equilibrium by application of an electromagneticpulse of appropriate frequency (RF pulse) while the correspondingmagnetic field B₁ of this RF pulse extends perpendicular to the z-axis,so that the magnetization performs a precessional motion about thez-axis. The precessional motion describes a surface of a cone whoseangle of aperture is referred to as flip angle. The magnitude of theflip angle is dependent on the strength and the duration of the appliedelectromagnetic pulse. In the case of a so-called 90° pulse, themagnetization is deflected from the z axis to the transverse plane (flipangle 90°). The transverse magnetization and its variation can bedetected by means of receiving RF coils which are arranged and orientedwithin an examination volume of the MR device in such a manner that thevariation of the magnetization is measured in the directionperpendicular to the z-axis.

In order to realize spatial resolution in the body, constant magneticfield gradients extending along the three main axes are superposed onthe uniform magnetic field B₀, leading to a linear spatial dependency ofthe spin resonance frequency. The signal picked up in the receivingcoils then contains components of different frequencies which can beassociated with different locations in the body.

The signal data obtained via the receiving coils correspond to thespatial frequency domain and are called k-space data. The k-space datausually include multiple acquired k-space profiles (lines in k-space) ofdifferent phase encoding. Each k-space profile is digitized bycollecting a number of samples. A set of k-space data is converted to anMR image by means of Fourier transformation.

To sensitize MRI images to diffusion, instead of a homogeneous magneticfield, the homogeneity is varied linearly by a pulsed field gradient.Since precession is proportional to the magnet strength, the protonsbegin to precess at different rates, resulting in dispersion of thephase and signal loss. Another gradient pulse is applied after some timein the same magnitude but with opposite direction to refocus or rephasethe spins. The refocusing will not be perfect for protons that havemoved during the time interval between the pulses, due to the variationof magnitude of the pulse between previous and current position of theproton, and the signal measured by the MRI machine is reduced.

With reference to FIG. 1, a MR imaging system 20 comprising a MR imagingapparatus 1 is shown. The apparatus comprises superconducting orresistive main magnet coils 2 such that a substantially uniform,temporally constant main magnetic field B₀ is created along a z-axisthrough an examination volume. The device further comprises a set of(1st, 2nd, and—where applicable—3rd order) shimming coils 2′, whereinthe current flow through the individual shimming coils of the set 2′ iscontrollable for the purpose of minimizing B₀ deviations within theexamination volume.

A magnetic resonance generation and manipulation system applies a seriesof RF pulses and switched magnetic field gradients to invert or excitenuclear magnetic spins, induce magnetic resonance, refocus magneticresonance, manipulate magnetic resonance, spatially and otherwise encodethe magnetic resonance, saturate spins, and the like to perform MRimaging.

More specifically, a gradient amplifier 3 applies current pulses orwaveforms to selected ones of whole-body gradient coils 4, 5 and 6 alongx, y and z-axes of the examination volume. A digital RF frequencytransmitter 7 transmits RF pulses or pulse packets, via a send/receiveswitch 8, to a body RF coil 9 to transmit RF pulses into the examinationvolume. A typical MR imaging sequence is composed of a packet of RFpulse segments of short duration which, together with any appliedmagnetic field gradients, achieve a selected manipulation of nuclearmagnetic resonance signals. The RF pulses are used to saturate, exciteresonance, invert magnetization, refocus resonance, or manipulateresonance and select a portion of a body as an example of an object 10positioned in the examination volume. The MR signals are also picked upby the body RF coil 9.

For generation of MR images of limited regions of the object 10 or forscan acceleration by means of parallel imaging, a set of local array RFcoils 11, 12, 13 are placed contiguous to the region selected forimaging. The array coils 11, 12, 13 can be used to receive MR signalsinduced by body-coil RF transmissions.

The resultant MR signals are picked up by the body RF coil 9 and/or bythe array RF coils 11, 12, 13 and demodulated by a receiver 14preferably including a preamplifier (not shown). The receiver 14 isconnected to the RF coils 9, 11, 12 and 13 via send/receive switch 8.

A host computer 15 controls the shimming coils 2′ as well as thegradient pulse amplifier 3 and the transmitter 7 to generate any of aplurality of MR imaging sequences, such as echo planar imaging (EPI),echo volume imaging, gradient and spin echo imaging, fast spin echoimaging, and the like. For the selected sequence, the receiver 14receives a single or a plurality of MR data lines in rapid successionfollowing each RF excitation pulse. A data acquisition system 16performs analog-to-digital conversion of the received signals andconverts each MR data line to a digital format suitable for furtherprocessing. In modern MR devices the data acquisition system 16 is aseparate computer which is specialized in acquisition of raw image data.

Ultimately, the digital raw image data are reconstructed into an imagerepresentation by a reconstruction processor 17 which applies a Fouriertransform or other appropriate reconstruction algorithms, such as SENSEor GRAPPA in the field of parallel imaging. The MR image may represent aplanar slice through the patient, an array of parallel planar slices, athree-dimensional volume, or the like. The image is then stored in animage memory where it may be accessed for converting slices,projections, or other portions of the image representation intoappropriate format for visualization, for example via a video monitor 18which provides a man-readable display of the resultant MR image.

In the MR imaging system 20 shown in FIG. 1, the system further includesa fat suppressed diffusion image determination apparatus 100 accordingto an aspect of the present invention. The fat suppressed diffusionimage determination apparatus 100 is configured to determine a diffusionweighted magnetic resonance image (DWI) of the object 10. The fatsuppressed diffusion image determination apparatus 100 thus implements aspecific method of reconstructing an MR image, in particular a DWI withefficient fat suppression.

Fat suppressed diffusion image determination apparatus 100 is in thisexample, shown as integrated into the reconstruction processor 17 andconfigured to communicate with the data acquisition system 16 and thehost computer 15 of the MR imaging apparatus 1. However, in otherexamples, image determination apparatus 100 can also be providedindependent from MR imaging apparatus 1 and be provided in the form of,for instance, one or more computing units.

It should be noted that of course the fat suppressed diffusion imagedetermination apparatus 100 can, in this example, rely on standard oravailable processing methods known in the art of MRI as exemplifiedabove, which are, for instance, implemented in reconstruction processor17, data acquisition system 16 and/or host computer 15, without any needfor particularly and explicitly describing these methods with respect tothe fat suppressed diffusion image determination apparatus 100 itself.

The fat suppressed diffusion image determination apparatus 100 comprisesa diffusion reference image providing unit 110 for providing a diffusionreference MR image of the object 10, a fat image determination unit 120for determining a fat image from the diffusion reference MR image, adiffusion weighted image providing unit 130 for providing a diffusionweighted MR image of the object, and a fat suppressed imagedetermination unit 140 for determining a fat suppressed diffusionweighted MR image using a combination of the diffusion weighted MR imageand the fat image.

The diffusion reference image providing unit 110 is configured toprovide a diffusion reference MR image from storage or throughacquisition by MR imaging apparatus 1, wherein the diffusion referenceimage providing unit 110 can be configured to control the operation ofthe relevant units of the MR imaging apparatus 1 for this purpose. Thediffusion reference MR image can in one example be a non-diffusionweighted or b=0 image, i.e. an image with the diffusion coefficient bbeing equal to 0. In other examples, the diffusion reference MR imagecan also have a low or rather insignificant diffusion parameter b, forinstance not above 200 s/mm².

The b-value is a factor that reflects the strength and timing of thegradients used to generate diffusion-weighted images. The higher theb-value, the stronger the diffusion effects, wherein the term “b-value”is widely accepted and originates from Stejskal et al. “Spin diffusionmeasurements: spin echoes in the presence of time-dependent fieldgradient.” J Chem Phys 1965; 42(1):288-292, in which the pulsed gradientdiffusion method as an example for DWI pulse sequences is described. Inessence, the method and substantially all current DWI pulse sequencesrelies on two strong gradient pulses, wherein the b-value depends on thestrength, duration, and spacing of these pulsed gradients. A largerb-value is achieved with increasing the gradient amplitude and durationand by widening the interval between gradient pulses. A value of b=0 isconsidered to represent an image without diffusion weighting.

The diffusion weighted image providing unit 130 then provides adiffusion weighted MR image of the object 10, wherein the acquisitionunderlying the provided MR image is preferentially similar to theacquisition of the diffusion reference MR image despite the differingb-value, i.e. the diffusion weighting. The fat suppressed imagedetermination unit 140 then determines the fat suppressed diffusionweighted MR image using a combination of the diffusion weighted MRimage, i.e. the image provided by diffusion weighted image providingunit 130, and the fat image, i.e. the fat image separated from thediffusion reference MR image, determined by the fat image determinationunit 120 as detailed below.

The invention is built on the insight that fat has a very lowdiffusivity and that thus a fat image, i.e. an image substantially onlyindicative of fat, obtained from the b=0 or low b-value, i.e. diffusionsuppressed, image can be employed in determining a fat suppresseddiffusion weighted MR image, i.e. an image with b>>0, as will bedetailed in the following.

The fat image determination unit 120 is in this examples configured toseparate fat from water based on the provided diffusion reference MRimage of the object 10. Multiple approaches to separating fat from waterare known in the art of MRI. In this example, it is preferred that thediffusion reference MR image be in the form of a multi-shot acquired EPIimage, wherein the fat image is reconstructed using a field map forwater/fat candidate selection. The need for multi TE acquisitions, inparticular for the acquisition of one or more diffusion weighted MRimages, can thus be alleviated.

Preferably, the diffusion reference MR image is a multi-shot EPI image.of which a combination of the multiple shots is reconstructed to complexvalued image data.

The EPI image is preferably acquired using multiple shots in the form ofa (partial) parallel imaging (PPI), wherein the fat image is thenfurther preferentially reconstructed based on the combined multi-shotacquisition without the application of the in-plane undersamplingreconstruction, e.g. using SENSE or any other employed PPI model. As analternative to the reconstruction using the field map, the SENSE waterand fat image separation according to Larkman et al. (ISMRM 2005, 505)can be employed on the combined multi-shot EPI acquisition.

The multiple shots of the diffusion reference MR image preferably coverthe entire k-space, wherein each shot has the same k-space trajectory,apart from a shift in phase encoding direction, as those acquired forthe diffusion weighted MR image, i.e. with b>0. Thereby, it can beassured that geometric deformation in each shot is the same.

A SENSE reduction factor of the provided diffusion reference MR image,likewise of the diffusion weighted MR images to be described in thefollowing, can be integer of not. In case of a non-integer SENSE factor,it is preferred to reconstruct with SENSE according to Larkman's methoddescribed above, wherein preferentially multiple averages of thediffusion reference MR image are added to reduce potential g-factorpenalty in the fat image. A different preferred approach is to increasethe FOV and SENSE factor with the same factor until the resulting SENSEfactor is an integer.

In summary, fat image determination unit 120 determines the fat imagefrom the diffusion reference MR image preferably by employing a parallelimaging approach to unfold water and fat signals on the basis of thechemical shift induced displacement between water and fat, which isknown a priori. Both the fat image and the diffusion reference MR imageare preferably provided as complex valued images to provide phasesensitivity.

The processing can be summarized and expressed in one example using ageneric formula.

$P = {\begin{pmatrix}W_{b = 0} \\F_{b = 0}\end{pmatrix} = {{\underset{\_}{\underset{\_}{H}}}_{b = 0} \cdot {\underset{\_}{\underset{\_}{m}}}_{b = 0}}}$

Two images—Water (W) and Fat (F)—are determined from a measured oracquired (folded) signal m for the b=0 acquisition and a combinationmatrix H.

Next, m is provided or acquired for the targeted b-factor b=B as thediffusion weighted MR image,

${\underset{\_}{\underset{\_}{m}}}_{b = B}.$

The fat suppressed image determination unit subtracts the folded fatsignal

${\underset{\_}{\underset{\_}{m}}}_{b = B}^{F} = {S_{F} \cdot F_{b = 0}}$

to obtain a water only (folded) diffusion weighted MR image

${\underset{\_}{\underset{\_}{m}}}_{b = B}^{W}.$

The water only, i.e. fat suppressed, diffusion weighted MR data can thenbe unfolded using

$P_{W} = {{\underset{\_}{\underset{\_}{H}}}_{b = B} \cdot {{\underset{\_}{\underset{\_}{m}}}_{b = B}^{W}.}}$

Using the water only signal processing prevents ill-conditioning of thereconstruction problem and further prevents the need for extension ofthe acquisition with phase navigators, as required for instance by theDIXON approach discussed in Burakiewicz et al. (DOI: 10.1002/mrm.25191,2014).

FIG. 2 shows schematically and exemplarily a flowchart of a fatsuppressed diffusion image determination method 200 according to theinvention.

The fat suppressed diffusion image determination method 200 is a methodfor determining a diffusion weighted magnetic resonance image (DWI) ofan object 10 and comprises the following steps.

In a step 210, a diffusion reference MR image of the object 10 isdetermined. The diffusion reference MR image of the object is, forinstance, determined by diffusion reference image providing unit 110 asdescribed above. Preferentially, the diffusion reference MR image is amulti-shot EPI image, which is reconstructed to complex valued imagedata.

In a step 220, a fat image is determined from the diffusion reference MRimage provided in step 210. The fat image is, for instance, determinedby fat image determination unit 120 as described above. Preferentially,the diffusion reference MR image is obtained with a particular reductionfactor and to determine an unfolded, complex valued fat image therefrom,SENSE unfolding is applied with water and fat in different locations inthe forward model knowing the “water-fat shift” from timing parametersof the EPI acquisition. The unfolded fat image so generated is thenforward folded with the applied reduction factor to determine the fatimage.

In a step 230, a diffusion weighted MR image of the object 10 isdetermined. The diffusion weighted MR image is, for instance, determinedby diffusion weighted image providing unit 130 as described above.Preferentially, the acquisition of the diffusion weighted MR imagediffers from the acquisition of the diffusion reference MR image in thediffusion encoding or weighting and number of shots, and the diffusionweighted MR image is provided as a folded, complex valued EPIreconstructed image. In practice, multiple diffusion weighted MR imageswith different diffusion parameters b will be provided.

Finally, in a step 240, a fat suppressed diffusion weighted MR image isdetermined using a combination of the diffusion weighted MR imageprovided in step 230 and the fat image determined in step 220. The fatsuppressed diffusion weighted MR image is, for instance, determined byfat suppressed image determination unit 140 as described above.Preferentially, the fat image is subtracted from the diffusion weightedMR image. It can be beneficial to employ a scaling parameter, which willoptimize the result of the subtraction by accounting for small signalchanges in the fat signal. Advantageously, an automatic determinationcould be done by identifying fat signal shifted relative to the anatomyand not overlapping with anatomy. Based on the result of thesubtraction, since the fat image has been removed from the diffusionweighted MR image, the residual diffusion-encoded water signal isreconstructed using SENSE. This can also be referred to as a water onlyunfolding.

The invention enables to achieve a fat suppressed diffusion weighted MRimage with minimal impact on g-factor behaviour and scan time comparedto a standard diffusion scan.

The order of steps is not limited to the order shown in FIG. 2 and, forinstance, step 230 can be performed prior to step 220, i.e. thediffusion weighted MR image can be provided before the fat image isdetermined based on the diffusion reference image.

Other variations to the disclosed embodiments can be understood andeffected by those skilled in the art in practicing the claimedinvention, from a study of the drawings, the disclosure, and theappended claims.

In the claims, the word “comprising” does not exclude other elements orsteps, and the indefinite article “a” or “an” does not exclude aplurality.

A single unit, component or device may fulfill the functions of severalitems recited in the claims. The mere fact that certain measures arerecited in mutually different dependent claims does not indicate that acombination of these measures cannot be used to advantage.

1. A fat suppressed diffusion image determination apparatus fordetermining a diffusion weighted magnetic resonance image (DWI) of anobject, the apparatus comprising: a diffusion reference image providingunit for providing a diffusion reference MR image of the object using aparallel imaging method, a fat image determination unit for determininga fat image from the diffusion reference MR image, a diffusion weightedimage providing unit for providing a diffusion weighted MR image of theobject, a fat suppressed image determination unit for determining a fatsuppressed diffusion weighted MR image using a combination of thediffusion weighted MR image and the fat image, wherein the diffusionreference image providing unit is configured to provide a foldedrepresentation of the diffusion reference MR image of the object,wherein the fat image determination unit is configured to determine anunfolded fat image by decomposing fat and water from the representationof the diffusion reference MR image, and to determine a foldedrepresentation of the decomposed fat components as the fat image.
 2. Theapparatus of claim 1, wherein the diffusion reference image providingunit is configured to provide the diffusion reference MR image acquiredwith a diffusion parameter of at most 200 s/mm², preferably at most 100s/mm², and most preferably at most 50 s/mm².
 3. The apparatus of claim1, wherein the diffusion weighted image providing unit is configured toprovide a plurality of diffusion weighted MR images of the object withrespective different diffusion parameters, wherein the fat suppressedimage determination unit is configured to provide a plurality of fatsuppressed diffusion weighted MR images for each of the plurality ofdiffusion weighted MR images using the fat image.
 4. The apparatus ofclaim 1, wherein the diffusion weighted image providing unit isconfigured to provide the MR images using a parallel imaging method. 5.The apparatus of claim 4, wherein the parallel imaging method comprisesSENSE.
 6. The apparatus of claim 1, wherein the fat image determinationunit is configured to determine a complex valued fat image and whereinthe diffusion reference image providing unit is configured to provide acomplex valued diffusion reference MR image.
 7. The apparatus of claim1, wherein the diffusion reference image providing unit is configured toprovide the diffusion reference MR image using multiple shots forcovering the entire k-space, wherein the multiple shots have similark-space trajectories, respectively, wherein the k-space trajectories ofthe multiple shots have a respectively different shift in phase encodingdirection.
 8. The apparatus of claim 7, wherein the diffusion referenceimage providing unit is configured to perform an echo planar imagingreconstruction to complex valued image data, wherein the fat imagedetermination unit is configured to determine the fat image using aSENSE separation of water and fat using the echo planar imagingreconstructed complex valued image data.
 9. The apparatus of claim 8,wherein the diffusion reference image providing unit is configured toprovide the diffusion reference image with a particular SENSE reductionfactor.
 10. The apparatus of claim 9, wherein the fat imagedetermination unit is configured to add multiple averages of thediffusion reference image to the fat image.
 11. The apparatus of claim1, wherein the diffusion weighted image providing unit is configured toprovide the diffusion weighted MR image before unfolding, wherein thefat suppressed image determination unit is configured to subtract thefat image from the folded diffusion weighted MR image and to determinethe fat suppressed diffusion weighted MR image by unfolding the fatimage subtracted folded diffusion weighted MR image.
 12. A magneticresonance imaging system for imaging an object, wherein the magneticresonance imaging system comprises a magnetic resonance scannerconfigured to acquire a diffusion reference MR image and a diffusionweighted MR image of the object, and a fat suppressed diffusion imagedetermination apparatus according to claim
 1. 13. A fat suppresseddiffusion image determination method for determining a diffusionweighted magnetic resonance image (DWI) of an object, the fat suppresseddiffusion image determination method comprising: providing a diffusionreference MR image of the object using a parallel imaging method and afolded representation of the diffusion reference MR image of the object,determining a fat image from the diffusion reference MR image and anunfolded fat image by decomposing fat and water from the representationof the diffusion reference MR image, and determining a foldedrepresentation of the decomposed fat components as the fat image,providing a diffusion weighted MR image of the object, and determining afat suppressed diffusion weighted MR image using a combination of thediffusion weighted MR image and the fat image
 14. A computer program forcontrolling a fat suppressed diffusion image determination apparatus asdefined in claim 1, the computer program comprising program code storedon a non-transitory computer readable medium for causing the fatsuppressed diffusion image determination apparatus to carry out the fatsuppressed diffusion image determination method as defined in claim 13,when the computer program is run on the magnetic resonance imagingapparatus.